Load balancing in a cellular telecommunication network

ABSTRACT

A method and apparatus for controlling a Network Load Balancing (NLB) algorithm that balances a traffic load between multiple downlink (DL) sectors in a cellular telecommunication network. A Connection Integrity Preservation (CIP) algorithm, which runs on top of the NLB algorithm in the Radio Network Controller/Base Station Controller (RNC/BSC), minimizes the risk of degrading network performance due to NLB offload decisions. The CIP algorithm may override an NLB offload decision, for example, if there have been too many offload failures, there are no target DL sectors available to acquire an offloaded Access Terminal (AT), or the offloaded AT is not acquired within a threshold time period. The CIP algorithm ensures required metrics are collected, and minimizes the impact on RNC/BSC processing due to Routing Update messages needed to make offload decisions. The invention enables the NLB algorithm to realize its potential without negative side-effects.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/451,235 filed Mar. 10, 2011; U.S. Provisional PatentApplication No. 61/451,245 filed Mar. 10, 2011; and U.S. ProvisionalPatent Application No. 61/451,545 filed Mar. 10, 2011, the disclosuresof which are fully incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

NOT APPLICABLE

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTINGCOMPACT DISC APPENDIX

NOT APPLICABLE

BACKGROUND

The present invention relates to cellular telecommunication systems.More particularly, and not by way of limitation, the present inventionis directed to an apparatus and method for balancing a traffic loadbetween multiple radio cells.

In existing 1x Evolved-Data Optimized (1x-EV-DO) advanced networks, aNetwork Load Balancing (NLB) algorithm provides methods of off-loadingtraffic from more loaded radio cells to less loaded radio cells bymanipulating allowed forward/downlink Base Transceiver Station (BTS)sectors from an Active Set (A_SET), for every active over-the-airconnection. The NLB algorithm utilizes the measured load on BTS sectorsin the A_SET and forces an Access Terminal (AT) to get service from aless loaded sector. This is done by having the Radio Network Controller(RNC) or Base Station Controller (BSC) unlock a Data Rate Control Lock(DRCLock) bit in a serving downlink (DRCLock=0), which forces the AT toselect a different serving downlink sector among the pilots from theA_SET.

Although helping to balance the traffic load and improve capacity andindividual user throughput, the NLB algorithm may degrade existing radioconnection Key Performance Indicators (KPIs) for several reasons. First,since the algorithm moves ATs from preferred downlink sectors topossibly less optimum sectors, this may lead to degraded connection lossstatistics. The connection drop rate is an important KPI that networkoperators use to measure overall health of the over-the-airconnectivity. Degradation of this KPI could invalidate capacity gainsand prevent successful NLB implementation.

Second, the NLB algorithm requires periodic Route Update reports fromthe ATs so that the algorithm can consider current RF signal qualitywhen making its balancing decisions. There is no option to set periodicreporting in 1x-EV-DO, so Route Update Messages have to be solicited toget the reports. This puts additional strain on the call processingentity at the RNC, and needs to be minimized without compromising theability of the NLB algorithm to gauge the signal strengths offorward/downlink pilots in the A_SET.

Third, the NLB algorithm must have a correct set of NLB metrics todetermine whether moving a given AT from one downlink sector to anotherwill do more harm than good due to adverse effects on other ATs. Withoutmechanisms for obtaining these metrics, the NLB algorithm may degradeSignal-to-Noise Ratio (SNR) and Quality of Service (QoS) for largernumbers of ATs.

SUMMARY

The present invention provides solutions to the above-mentionedproblems. In one embodiment, the invention provides an RNC algorithmreferred to herein as the Connection Integrity Preservation (CIP)algorithm, which runs on top of the NLB algorithm and provides steps,measures, and checks that minimize the risk of degrading over-the-airconnectivity KPIs due to forced offload decisions, and enable the NLBalgorithm to realize its potential without negative side-effects. TheCIP algorithm runs for each active connection for which NLB offloadingdecisions have been made, and ensures that all forward/downlink serversthat have been force-barred by the NLB can be made available again, whenthe CIP algorithm detects that the connection may drop. In addition,basic protections are put in place to prevent the NLB algorithm frommaking repetitive offload decisions if there is a history of previouslyfailed offload attempts during the same connection.

In another embodiment, the present invention also ensures the NLBalgorithm receives the Route Update Messages it needs to gauge thesignal strengths of forward/downlink pilots in an AT's A_SET, whileminimizing the signaling and call processing load on the RNC. Thisembodiment ensures that a minimum threshold level of contention ispresent in the current forward/downlink sector before the RNC sendsperiodic Route Update Request messages and processes the responses tothose messages. This minimizes the call processing impact by ensuringthat Route Update Requests and Route Update Messages are sent only whenrequired. Furthermore, this embodiment provides procedures for unlockingthe DRCLock bit not only on the serving forward/downlink sector, butalso on other loaded sectors so that the AT is forced to the leastloaded sector.

In a further embodiment, the invention also prevents excessive sectorswitching where the capacity benefit (in terms of metric differencebetween the current serving sector and a target sector) is too small.Likewise, the invention may prevent a forced offload when the adverseimpact on other ATs is determined to be too great.

In a particular embodiment, the present invention is directed to amethod of controlling an NLB algorithm that balances a traffic loadbetween multiple downlink (DL) sectors in a cellular telecommunicationnetwork, wherein the method is performed by a processor associated witha Radio Network Controller/Base Station Controller (RNC/BSC). The methodincludes the steps of detecting that the NLB algorithm has decided tooffload a given Access Terminal (AT) from a current serving DL sector toone of a plurality of target DL sectors in the given AT's active set dueto a high traffic load in the current serving DL sector; determining acount of previous attempts to offload the given AT from the currentserving DL sector that failed because the given AT was not acquired byany of the target DL sectors; and when the count of failed previousattempts to offload the given AT from the current serving DL sectorexceeds a threshold amount, overriding the NLB algorithm decision tooffload the given AT from the current serving DL sector.

In further embodiments, the method may include overriding the NLBalgorithm decision to offload the given AT from the current serving DLsector when an earlier attempt to offload the given AT failed and thetime period since the failure has been too short. Additionally, theinvention may override the NLB algorithm decision to offload the givenAT from the current serving DL sector when there are no target DLsectors in the given AT's active set that are currently allowed toacquire the given AT.

In another embodiment, the invention is directed to an apparatus in anRNC/BSC for controlling an NLB algorithm that balances a traffic loadbetween multiple DL sectors in a cellular telecommunication network. Theapparatus includes a processor and a non-transitory memory for storing aConnection Integrity Preservation (CIP) algorithm. When the processorexecutes the CIP algorithm, the apparatus is caused to perform the stepsof detecting that the NLB algorithm has decided to offload a given ATfrom a current serving DL sector to one of a plurality of target DLsectors in the given AT's active set due to a high traffic load in thecurrent serving DL sector; determining a count of previous attempts tooffload the given AT from the current serving DL sector that failedbecause the given AT was not acquired by any of the target DL sectors;and when the count of failed previous attempts to offload the given ATfrom the current serving DL sector exceeds a threshold amount,overriding the NLB algorithm decision to offload the given AT from thecurrent serving DL sector.

In another embodiment, the invention is directed to an apparatus in anRNC/BSC for controlling an NLB algorithm that balances a traffic loadbetween multiple DL sectors in a cellular telecommunication network. Theapparatus includes a processor and a non-transitory memory for storing aCIP algorithm. When the processor executes the CIP algorithm, theapparatus is caused to perform the steps of determining whether a timeperiod since an earlier failed attempt to offload the given AT has beentoo short; and overriding the NLB algorithm decision to offload thegiven AT from the current serving DL sector when the time period sincethe earlier failed attempt has been too short.

In another embodiment, the invention is directed to an apparatus in anRNC/BSC for controlling an NLB algorithm that balances a traffic loadbetween multiple DL sectors in a cellular telecommunication network. Theapparatus includes a processor and a non-transitory memory for storing aCIP algorithm. When the processor executes the CIP algorithm, theapparatus is caused to perform the steps of determining whether thereare any target DL sectors in the given AT's active set that arecurrently allowed to acquire the given AT; and overriding the NLBalgorithm decision to offload the given AT from the current serving DLsector when there are no target DL sectors in the given AT's active setthat are currently allowed to acquire the given AT.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following section, the invention will be described with referenceto exemplary embodiments illustrated in the figures, in which:

FIGS. 1A-1B are portions of a flow chart illustrating the steps of anexemplary embodiment of the Connection Integrity Preservation (CIP)algorithm;

FIG. 2 is a flow chart illustrating the steps of an exemplary embodimentof a method of providing the NLB algorithm with the Route UpdateMessages it needs to gauge the signal strengths of forward/downlinkpilots in an AT's active set; and

FIG. 3 is a simplified block diagram of an exemplary embodiment of anapparatus in an RNC/BSC for controlling the NLB algorithm in accordancewith the teachings of the present invention.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention. Additionally, it should be understood that the invention canbe implemented in hardware or in a combination of hardware processor(s)and computer program instructions stored on a non-transitory storagemedium.

FIG. 1A is a flow chart illustrating the steps of an exemplaryembodiment of a first portion of the Connection Integrity Preservation(CIP) algorithm. At step 11, data is forwarded to the serving DL sector,if available. At step 12, the NLB algorithm determines that the activeDL sector is overloaded (and potentially other sectors as well) and agiven AT needs to be offloaded. At this point, the CIP algorithm makes acheck to determine whether the offload should be allowed. At step 13,the CIP algorithm determines whether there were any previous failedattempts to offload the given AT from the current serving DL sector.This may be done by determining whether there is a running OffLoadTimerand whether the count of previously failed offloads of the given AT isgreater than a configurable threshold THOLD_1. An “offload failure” isintended to refer to an event where a forced DRC unlock was performed tothe current serving DL sector, but no other radio node indicatedsuccessfully acquiring the AT that was offloaded within a configurableperiod of time controlled by a SWITCH_TO timer.

When the OffLoadTimer is >0 or the count of previously failed offloadsof the given AT is greater than THOLD_1, the method moves to step 14where the CIP algorithm overrules the NLB decision and preventsunlocking of the DRCLock bit on the current serving DL sector. Datacontinues to be forwarded to the serving DL sector. However, when theOffLoadTimer=0 and the count of previously failed offloads of the givenAT is less than THOLD_1, the method moves to step 15 where the CIPalgorithm causes the RNC to determine whether the DRCLock bit is set onany other sector/cell in the given AT's current A_SET, other than theones the NLB algorithm has indicated should be offloaded. In otherwords, the RNC determines whether there are any other DL sectors in theAT's A_SET that are allowed to acquire the AT. When there are none, themethod returns to step 14 where the CIP algorithm overrules the NLBdecision and prevents unlocking of the DRCLock bit on the currentserving DL sector. The purpose of this check is to prevent forcedoffloads when there are no other radio nodes in the current A_SETcapable of acquiring the offloaded AT. When there is at least one othersector in the current A_SET with the DRCLock bit set, the method movesto step 16 where the RNC honors the NLB algorithm decision and instructsthe serving DL sector (and any other sectors the NLB selected) toforce-unlock the DRCLock bit. At step 17, the flow of data to theserving DL sector is stopped and a SwitchTo timer is started (i.e.,acquisition of the new serving DL sector is expected to occur withinthis time-out period). At step 18, the RNC saves all force-unlockedsectors in a list for later referral, if needed.

FIG. 1B is a flow chart illustrating the steps of an exemplaryembodiment of a second portion of the CIP algorithm. After theforce-unlock action shown in FIG. 1A, the RNC expects to receive anindication from one of the serving radio nodes, other than the one thatwas force-unlocked, that the given AT has been acquired within apredefined time period. Thus at step 21, the RNC determines whether anew radio node acquires the offloaded AT within the SWITCH-TO timeperiod defined by the SwitchTo timer. If not, the CIP algorithm assumesthe AT was not successful in switching to the new DL and henceconnection integrity is in question. At step 22, the CIP algorithmresets the SwitchTo timer. At step 23, the CIP algorithm instructs allsectors in the force unlocked list to lock the DRCLock bit again, andthe CIP algorithm clears the list. At step 24, the CIP algorithm startsan OffloadTimer against the previously serving DL sector. TheOffloadTimer temporarily prevents switching for a configurable timeperiod defined by the variable OFFLOAD_BARRED (for example for 500 ms).When the OFFLOAD_BARRED time period expires, OffloadTimer resets to 0and NLB switching is allowed again, unless the failure counter isgreater then or equal to THOLD_1. At step 25, the CIP algorithmincrements an OffloadFailed counter. The method then returns to thestart of FIG. 1A.

The OffloadTimer timer and the OffloadFailed counter are utilized toprevent successive NLB-induced offloads that have recently failed tocomplete in the same active connection.

When the AT's DRCCover change is successfully decoded by the new radionode and the RNC gets that information within the SWITCH-TO time period,the method moves from step 21 to step 26 where the CIP algorithmstop/resets the CIP's SwitchTo timer. At step 27, the CIP algorithmresets the OffloadFailed counter. The method then moves to step 28 wherethe CIP algorithm causes the RNC to monitor whether there is ever anevent where acquisition on the serving DL is lost for a time greaterthan SWITCH_TO. If not, the RNC continues forwarding data to the currentserving DL sector at step 29. However, if the acquisition on the servingDL is lost for a time greater than SWITCH_TO, this indicates an eventwhere connection may be potentially lost. Thus the method moves to step30 where the CIP algorithm determines whether there are any entries inthe force-unlock list and if so, instructs all sectors in theforce-unlock list to lock the DRCLock bit and clear the list. The methodthen returns to the start of FIG. 1A.

It should also be noted that the OffloadTimer and OffloadFailed counterare initialized to zero (0) at every DL change; the CIP configurables,OFFLOAD_BARRED time, and SWITCH_TO time are set large enough tocircumvent a 1x Tune-away; the CIP algorithm runs separately for eachactive connection; the CIP flow charts illustrated in FIGS. 1A-1B depictmanagement of a given active connection in the context of NLB algorithmdecisions; and the radio node gets cleared from the force-unlocked listif it drops out of the A_SET.

Thus, the above embodiment implements protective measures to prevent aconnection drop in case an NLB-induced offload fails. Artificialunlocking of the best active set server is made revocable in case otherradio nodes lose acquisition on the AT at any point after artificialunlock occurs, assuming that the force-unlocked radio node is still inthe A_SET. Hysteresis is built into the CIP algorithm to protect againstsuccessive failed offload attempts.

In another embodiment, the present invention also ensures the NLBalgorithm receives the Route Update Messages it needs to gauge thesignal strengths of forward/downlink pilots in the A_SET, whileminimizing the signaling and call processing load on the RNC. Thisembodiment ensures that a minimum threshold level of contention ispresent in the current forward/downlink sector before the RNC sendsperiodic Route Update Request messages and processes the responses tothose messages. This minimizes the call processing impact by ensuringthat Route Update Requests and Route Update Messages are sent only whenrequired. Furthermore, this embodiment provides procedures for unlockingthe DRCLock bit not only on the serving forward/downlink sector, butalso on other loaded sectors so that the AT is forced to the leastloaded sector.

In the discussion that follows, “Neff” is the long-term measure of thenumber of users with non-empty queues. Each BTS sends Neff to the RNC,and the RNC queries the AT for a Route Update message only when usercontention/Neff at the current serving DL sector exceeds a configuredminimum threshold (Neff_min_Threshold). The RNC then starts toperiodically send a Route Update Request to the AT (with a period ofRuR_periodicity). The AT responds by sending a Route Update message withthe Signal-to-Interference (Ec/lo) ratio of pilots in the A_SET.

The RNC then uses the metrics Neff, DRCLock, and Ec/lo from the A_SETfor its load balancing decision. The RNC compares the metrics amongA_SET members and determines whether a legacy AT is a candidate for NLB.If so, the RNC offloads the AT from its capacity constrained sector bysetting the DRCLock bit to 0 (out-of-lock) forbidding the AT to use thatsector.

FIG. 2 is a flow chart illustrating the steps of an exemplary embodimentof a method of providing the NLB algorithm with the Route UpdateMessages it needs to gauge the signal strengths of forward/downlinkpilots in the A_SET. At step 41, the RNC determines whether the numberof sectors in the A_SET is greater than one. If not, the RNC continuesto monitor the A_SET until there are multiple sectors in the A_SET. Themethod then moves to step 42 and determines whether the Neff value ofthe current serving DL sector (Neff,dl) is greater thanNeff_min_Threshold. If not, the method returns to step 41. If yes, themethod moves to step 43 where the RNC sends a Route Update Requestmessage to the AT and then processes the Route Update message the ATsends in response to determine an NLB metric for each candidate DLsector in the A_SET. The metric may be, for example, (Pilot_SINR(dB)−Neff (dB)), in which case the higher the metric value, the moresuitable an offload candidate becomes. At step 44, the RNC maintains anNLB metric list for the A_SET members, ranked from highest to lowest.

At step 45, it is determined whether there is at least one sector “i”for which (NLB, dl−NLB, i) is greater than Delta_Threshold. If not, themethod returns to step 41. If yes, the method moves to step 46 where itis determined whether there are sectors “k” for which (NLB,dl−NLB, k) isless than Delta_Threshold. If not, the method moves to step 47 where theRNC selects only the serving DL sector for DRCLock bit unlocking. Ifyes, the method moves to step 48 where the RNC selects for DRCLock bitunlocking, the serving DL sector and all other sector(s) with(NLB,dl−NLB, k) less than Delta_Threshold.

In another embodiment, instead of using minimum contention/Neff as thetrigger to start sending periodic Route Update Request messages, thetrigger may be a minimum effective physical layer, Tput, defined as DRCdivided by Neff (hereafter, DRC_eff_min). Based on reports ofcontention/Neff from the radio nodes and the periodically reported Ec/lofor the pilots in the A_SET, applying the NLB metric will decide whetheror not a forced sector switch from a serving sector to a target sectoris needed. However if the difference between the metric measurementsfrom the serving and target sectors is less than a configuredDelta_Threshold, the forced sector switch is not performed.Additionally, the forced sector switch is not performed when thecontention/Neff level of the target sector is higher than a configuredmaximum threshold (Neff_max_Threshold).

In cases where more than two sectors are part of the A_SET, there may bea need to unlock the DRCLock bit on additional sectors besides thecurrent serving DL sector. For example, there may be three pilots P1,P2, and P3 in the A_SET, with P1 having the highest reported Ec/lo, P2having the second highest reported Ec/lo, and P3 having the lowestreported Ec/lo. P3, however, has the best NLB metric and P2 has theworst NLB metric. The current serving sector maps to P1. In this case,there should be a switch from P1 to P3 as the target serving sector, butin order to do that, the DRCLock bit on both P1 and P2 must be unlockedto prevent the AT from accessing P2.

In this manner, the present invention minimizes the call processingimpact due to sending Route Update Request messages and receiving RouteUpdate messages in response by sending Route Update Request messagesonly when the RNC determines the need for it. The invention providespotential additional capacity gain in the multi-sector SHO scenario bypreventively unlocking the DRCLock bit not only on the serving DLsector, but also on other loaded sectors, targeting the least loadedsector. Additionally, the invention prevents excessive sector switchingwhen the capacity benefit (in terms of metric difference between thecurrent serving sector and the target sector) is too small.

As noted above, the NLB algorithm must have a correct set of NLB metricsto determine whether moving a given AT from one downlink sector toanother will do more harm than good due to adverse effects on other ATs.Without mechanisms for obtaining these metrics, the NLB algorithm maydegrade the SNR and QoS for larger numbers of ATs. Using a correct setof metrics is essential to ensure that the load balancing gains arerealized.

1x Evolved-Data Optimized (1x-EV-DO) advanced networks support both bestefforsts (BE) and delay sensitive (DS) flows. DS flows are given higherpriority than BE flows during packet scheduling. Hence a DS flow activeon a sector will impact the perceived throughput for a BE flow. If an ATis offloaded to a sector with a large number of DS flows, the AT mayexperience worse performance. Currently there is no consideration ofimpact due to DS flows. Also offloading a given AT may improve the userexperience for that AT but degrade service for all other ATs. Currently,the NLB algorithm does not consider the impact on the performance ofother users resulting from the network balancing. In an furtherembodiment of the present invention, these problems are also addressed.

In this embodiment, during the determination of AT offloading, the NLBalgorithm computes an NLB metric to determine the sector to whichtraffic should be offloaded. This embodiment ensures that additionalmetrics are calculated, which consider the impact due to DS flows.Further the embodiment takes into account the performance of other usersand overall sector throughput performance, and ensures it is notseverely degraded.

The NLB algorithm is executed per AT. Based on the sector load (Neff),an AT's pilot strength (Ec/lo), and the A_SET, a target sector isselected to offload the AT. An NLB Metric (NLB_(M)) has the followingrelation to each of these metrics:

-   -   The lower the sector load, the higher the likelihood of        selecting the target sector since it would provide a better user        throughput. Neff is normalized by the total usable slots for        unicast traffic.

NLB_(M)∝1/Neff

-   -   The higher the pilot strength of the target AT, the higher the        likelihood of having a successful offload.

NLB_(M)∝(Ec/lo)_(AT)

In this embodiment, the NLB algorithm calculates an enhanced NLB Metric(NLB_(M Enh)). The NLB algorithm first considers a DS flow factor. SinceDS flows are given Entry higher priority in the network, having more DSflows implies fewer slots available to share between non-delay sensitive(e.g., BE) flows. This in turn will result in lower throughput. Hence asector having high number of DS flows may not be a good candidate towhich to offload ATs. In general, the relation between the enhancedNLB_(M Enh) and DS flows can be summarized as:

NLB_(MEnh)∝(1/DS_flow_factor)

The DS flow factor may be incorporated into the NLB calculations inseveral ways. As an example, it may be incorporated based on slotutilization. In this embodiment, the slots used by the DS flow areremoved. Since during slot contention between a DS flow and BE flows itis highly likely that the DS flow will be given priority, removing theslots used by DS flows so as to eliminate this prioritization againstother resource-contending users. The slot utilization can be a filterednumber per sector averaged over a user-defined interval. Thisinformation should be available at the BTS level but may need to beextracted using an Application Programming Interface (API) and providedas an input to the NLB algorithm.

NLB_(MEnh)∝(1/DS_flows_slot_usage)

The NLB algorithm also considers a Effective SNR Delta (SNR_(delta) _(—)_(eff)). When an AT is offloaded from a serving sector to a targetsector, the decision is usually intended to improve the offloaded user'sperformance. The metrics do not consider the impact on the userexperience of other users in the target sector. Often, however,offloading has a negative impact on other users in the target sector.SNR_(delta) _(—) _(eff) may be used to control the degree ofdegradation.

SNR_(delta) _(—) _(eff) is the effective change in network SNR due to anAT being offloaded. It should be noted that other metrics mayalternatively be utilized in this role. For example, SNR may be replacedwith other quantities such as Ec/Io or C/I, which also provide arepresentation of the user's DL channel. SNR_(delta) _(—) _(eff) is usedas a representation of potential change in user/network throughput dueto AT offloading. This can be viewed as a predictive metric and can bebiased towards a given AT or sector throughput improvements. It may alsohelp minimize AT offloadings that are less beneficial.

The following definitions are provided:

SNR_(t)=Average_SNR_(target) _(—) _(sector) _(—) _(before) _(—) _(AT)_(—) _(Offloading) represents the average of SNR across all users in thetarget sector

SNR_(s)=Average_SNR_(source) _(—) _(without) _(—) _(given) _(—) _(AT)represents the average of SNR across all users in the source sector notincluding given AT

SNR_(AT)=Average_SNR_(given) _(—) _(AT)

SNR_(delta) _(—) _(eff) =A*Average (SNR_(t), SNR_(AT))−B*SNR_(s)

where A and B are constants. For example average number of activeconnections in the sector

The SNR_(delta) _(—) _(eff) measurement may be incorporated into the NLBcalculations in several ways. First, it may be implemented as a decisionswitch. In this embodiment, the NLB algorithm evaluates SNR_(delta) _(—)_(eff) for each target sector in the given AT's A_SET (A_SET). AnSNR_(delta) _(—) _(eff) _(—) _(threshold) datafill is introduced, whichdetermines the maximum allowable change in network SNR. If sectorSNR_(delta) _(—) _(eff) is below the SNR_(delta) _(—) _(eff) _(—)_(threshold), the sector is considered for NLB. Otherwise, the sector isnot considered as a candidate sector for offload. The NLB algorithm thenuses all other NLB metrics to determine a winning sector for the AT fromthe candidate sectors for NLB.

Second, the SNR_(delta) _(—) _(eff) measurement may be implemented as apriority switch. In this embodiment, the NLB algorithm evaluatesSNR_(delta) _(—) _(eff) for each target sector in the given ATs A_SET.The A_SET is ordered in the order of least SNR_(delta) _(—) _(eff). TheNLB algorithm then uses other NLB metrics to determine the winningsector for the AT.

Third, the SNR_(delta) _(—) _(eff) measurement may be implemented byintegrating it with NLB metrics. In the following equation, ‘Function’combines the two indicated quantities. Weighting coefficients α and βmay be utilized to adjust the metric towards a given AT's performance orall user/sector performance.

${NLB}_{M} = {{Function}\left( {{\alpha*\frac{\left( {{Ec}/{Io}} \right)_{{AT}\; 1}}{N_{eff}}},{\beta*{SNR}_{delta\_ eff}}} \right)}$

Note that the first term in the parentheses is the metric thatdetermines the performance of a given AT. The second term is the metricthat determines the performance of other users or the sector.

FIG. 3 is a simplified block diagram of an exemplary embodiment of anapparatus in an RNC/BSC 51 for controlling the NLB algorithm 52. Aprocessor 53 executes computer program instructions stored in anon-transitory memory 54. The computer program instructions include theNLB algorithm 52 and the CIP algorithm 55 running on top. When theprocessor executes the NLB and CIP algorithms, the apparatus performsthe various steps illustrated in FIGS. 1A-1B and FIG. 2.

For completeness, the RNC/BSC 51 is also shown to include a BTSinterface 56 for communicating data as well as various metrics,requests, responses, and instructions with a plurality of BTSs 57 a-57 nlabeled BTS-A through BTS-N. BTSs 57 a-57 n control a number of DLsectors 58 a-58 n, respectively.

As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a wide range of applications. Accordingly, the scope of patentedsubject matter should not be limited to any of the specific exemplaryteachings discussed above, but is instead defined by the followingclaims.

1. A method of controlling a Network Load Balancing (NLB) algorithm that balances a traffic load between multiple downlink (DL) sectors in a cellular telecommunication network, the method being performed by a processor associated with a Radio Network Controller/Base Station Controller (RNC/BSC), the method comprising the steps of: detecting that the NLB algorithm has decided to offload a given Access Terminal (AT) from a current serving DL sector to one of a plurality of target DL sectors in the given AT's active set due to a high traffic load in the current serving DL sector; determining a count of previous attempts to offload the given AT from the current serving DL sector that failed because the given AT was not acquired by any of the target DL sectors; and when the count of failed previous attempts to offload the given AT from the current serving DL sector exceeds a threshold amount, overriding the NLB algorithm decision to offload the given AT from the current serving DL sector.
 2. The method according to claim 1, further comprising the steps of: determining whether a time period since an earlier failed attempt to offload the given AT has been too short; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when the time period since the earlier failed attempt has been too short.
 3. The method according to claim 1, further comprising the steps of: determining whether there are any target DL sectors in the given AT's active set that are currently allowed to acquire the given AT; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when there are no target DL sectors in the given AT's active set that are currently allowed to acquire the given AT.
 4. The method according to claim 1, further comprising, when the count of failed previous attempts to offload the given AT from the current serving DL sector does not exceed the threshold amount, the steps of: allowing the NLB algorithm decision to offload the given AT from the current serving DL sector, wherein the given AT is offloaded by unlocking a Data Rate Control Lock (DRCLock) bit in the current serving DL sector, which forces the given AT to select a different serving DL sector from the target DL sectors in the given AT's active set; determining whether the given AT is acquired by one of the target DL sectors within a threshold time period; and when the given AT is not acquired by one of the target DL sectors within the threshold time period, instructing all DL sectors in the given AT's active set having unlocked DRCLock bits to lock their DRCLock bits, thereby stopping all attempts to offload the given AT by the NLB algorithm.
 5. The method according to claim 4, further comprising the steps of: starting an offload barring timer for the current serving DL sector to define a time period during which the NLB algorithm is prevented from offloading the given AT; and incrementing the count of failed attempts to offload the given AT from the current serving DL sector.
 6. The method according to claim 4, further comprising the steps of: when the given AT is acquired by one of the target DL sectors within the threshold time period, determining whether the acquiring DL sector loses connection with the given AT for a defined time period; and when the acquiring DL sector loses connection with the given AT for the defined time period, instructing all DL sectors having unlocked DRCLock bits to lock their DRCLock bits, thereby stopping all attempts to offload the given AT by the NLB algorithm.
 7. The method according to claim 1, further comprising, when the count of failed previous attempts to offload the given AT from the current serving DL sector does not exceed the threshold amount, allowing the NLB algorithm decision to offload the given AT from the current serving DL sector.
 8. The method according to claim 7, wherein the given AT is offloaded by unlocking a Data Rate Control Lock (DRCLock) bit in the current serving DL sector, which forces the given AT to select a different serving DL sector from the target DL sectors in the given AT's active set, and the method further comprises the steps of: determining which other DL sectors in the given AT's active set also have high traffic loads; and unlocking the DRCLock bit not only on the current serving DL sector, but also on the other highly loaded DL sectors, thereby forcing the given AT to move a target DL sector that is not highly loaded.
 9. The method according to claim 7, further comprising the steps of: determining whether the offload will move the given AT to a target DL sector having a capacity that is greater than the capacity of the current serving DL sector by less than a threshold amount; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when the capacity of the target DL sector is greater than the capacity of the current serving DL sector by less than the threshold amount, thereby preventing excessive sector switching where a capacity benefit is too small.
 10. The method according to claim 7, further comprising the steps of: determining whether the offload will move the given AT to a target DL sector in which performance for a number of ATs in the target DL sector greater than a threshold amount will be adversely impacted by moving the given AT to the target DL sector; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when performance for a number of ATs in the target DL sector greater than the threshold amount will be adversely impacted by moving the given AT to the target DL sector.
 11. The method according to claim 1, further comprising the steps of: determining a level of contention in the current serving DL sector; and controlling the RNC/BSC to send periodic Route Update Request messages to ATs in the current serving DL sector only when the determined level of contention in the current serving DL sector exceeds a minimum threshold level of contention.
 12. An apparatus in a Radio Network Controller/Base Station Controller (RNC/BSC) for controlling a Network Load Balancing (NLB) algorithm that balances a traffic load between multiple downlink (DL) sectors in a cellular telecommunication network, the apparatus comprising: a processor; and a non-transitory memory for storing a Connection Integrity Preservation (CIP) algorithm; wherein when the processor executes the CIP algorithm, the apparatus is caused to perform the steps of: detecting that the NLB algorithm has decided to offload a given Access Terminal (AT) from a current serving DL sector to one of a plurality of target DL sectors in the given AT's active set due to a high traffic load in the current serving DL sector; determining a count of previous attempts to offload the given AT from the current serving DL sector that failed because the given AT was not acquired by any of the target DL sectors; and when the count of failed previous attempts to offload the given AT from the current serving DL sector exceeds a threshold amount, overriding the NLB algorithm decision to offload the given AT from the current serving DL sector.
 13. The apparatus according to claim 12, wherein when the processor executes the CIP algorithm, the apparatus is caused to perform the additional steps of: determining whether a time period since an earlier failed attempt to offload the given AT has been too short; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when the time period since the earlier failed attempt has been too short.
 14. The apparatus according to claim 12, wherein when the processor executes the CIP algorithm, the apparatus is caused to perform the additional steps of: determining whether there are any target DL sectors in the given AT's active set that are currently allowed to acquire the given AT; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when there are no target DL sectors in the given AT's active set that are currently allowed to acquire the given AT.
 15. The apparatus according to claim 12, wherein when the count of failed previous attempts to offload the given AT from the current serving DL sector does not exceed the threshold amount, the processor causes the apparatus to perform the steps of: allowing the NLB algorithm decision to offload the given AT from the current serving DL sector, wherein the given AT is offloaded by unlocking a Data Rate Control Lock (DRCLock) bit in the current serving DL sector, which forces the given AT to select a different serving DL sector from the target DL sectors in the given AT's active set; determining whether the given AT is acquired by one of the target DL sectors within a threshold time period; and when the given AT is not acquired by one of the target DL sectors within the threshold time period, instructing all DL sectors in the given AT's active set having unlocked DRCLock bits to lock their DRCLock bits, thereby stopping all attempts to offload the given AT by the NLB algorithm.
 16. The apparatus according to claim 15, wherein when the given AT is acquired by one of the target DL sectors within the threshold time period, the processor causes the apparatus to perform steps of: determining whether the acquiring DL sector loses connection with the given AT for a defined time period; and when the acquiring DL sector loses connection with the given AT for the defined time period, instructing all DL sectors in the given AT's active set having unlocked DRCLock bits to lock their DRCLock bits, thereby stopping all attempts to offload the given AT by the NLB algorithm.
 17. The apparatus according to claim 12, wherein when the count of failed previous attempts to offload the given AT from the current serving DL sector does not exceed the threshold amount, the processor causes the apparatus to perform the steps of: allowing the NLB algorithm decision to offload the given AT from the current serving DL sector, wherein the given AT is offloaded by unlocking a Data Rate Control Lock (DRCLock) bit in the current serving DL sector, which forces the given AT to select a different serving DL sector from the target DL sectors in the given AT's active set; determining which other DL sectors in the given AT's active set also have high traffic loads; and unlocking the DRCLock bit not only on the current serving DL sector, but also on the other highly loaded DL sectors, thereby forcing the given AT to move a target DL sector that is not highly loaded.
 18. The apparatus according to claim 12, wherein when the count of failed previous attempts to offload the given AT from the current serving DL sector does not exceed the threshold amount, the processor causes the apparatus to perform the steps of: allowing the NLB algorithm decision to offload the given AT from the current serving DL sector; determining whether the offload will move the given AT to a target DL sector having a capacity that is greater than the capacity of the current serving DL sector by less than a threshold amount; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when the capacity of the target DL sector is greater than the capacity of the current serving DL sector by less than the threshold amount, thereby preventing excessive sector switching where a capacity benefit is too small.
 19. The apparatus according to claim 12, wherein when the count of failed previous attempts to offload the given AT from the current serving DL sector does not exceed the threshold amount, the processor causes the apparatus to perform the steps of: allowing the NLB algorithm decision to offload the given AT from the current serving DL sector; determining whether the offload will move the given AT to a target DL sector in which performance for a number of ATs in the target DL sector greater than a threshold amount will be adversely impacted by moving the given AT to the target DL sector; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when performance for a number of ATs in the target DL sector greater than the threshold amount will be adversely impacted by moving the given AT to the target DL sector.
 20. The apparatus according to claim 12, wherein when the processor executes the CIP algorithm, the apparatus is caused to perform the additional steps of: determining a level of contention in the current serving DL sector; and controlling the RNC/BSC to send periodic Route Update Request messages to ATs in the current serving DL sector only when the determined level of contention in the current serving DL sector exceeds a minimum threshold level of contention.
 21. An apparatus in a Radio Network Controller/Base Station Controller (RNC/BSC) for controlling a Network Load Balancing (NLB) algorithm that balances a traffic load between multiple downlink (DL) sectors in a cellular telecommunication network, the apparatus comprising: a processor; and a non-transitory memory for storing a Connection Integrity Preservation (CIP) algorithm; wherein when the processor executes the CIP algorithm, the apparatus is caused to perform the steps of: determining whether a time period since an earlier failed attempt to offload the given AT has been too short; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when the time period since the earlier failed attempt has been too short.
 22. An apparatus in a Radio Network Controller/Base Station Controller (RNC/BSC) for controlling a Network Load Balancing (NLB) algorithm that balances a traffic load between multiple downlink (DL) sectors in a cellular telecommunication network, the apparatus comprising: a processor; and a non-transitory memory for storing a Connection Integrity Preservation (CIP) algorithm; wherein when the processor executes the CIP algorithm, the apparatus is caused to perform the steps of: determining whether there are any target DL sectors in the given AT's active set that are currently allowed to acquire the given AT; and overriding the NLB algorithm decision to offload the given AT from the current serving DL sector when there are no target DL sectors in the given AT's active set that are currently allowed to acquire the given AT. 